A Catoptric Immersion lUuminator. Bij J. W. Stephenson. 37 



It will be found that this instrument will work through any 

 ordinary glass slip, gives a brilliant light, and, having no refracting 

 surface, is necessarily achromatic, whilst the spherical aberration, as 

 previously pointed out, is inconsiderable. 



If used with a dry lens of the highest power on a balsam- 

 mounted object, the light, unable to pass the upper surface of the 

 covering glass, is thrown back on the object, giving opaque illumi- 

 nation ; on the other hand, with dry objects adhering to the slide, 

 the well-known dark-ground illumination can be obtained with any 

 objective I have yet seen. 



